Download Introduction to Python Python can be simple Python Modules allow

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Python can be simple
Introduction to Python
#!/usr/bin/python
is more powerful than Tcl
applicable to larger systems development
print "hello world"
has cleaner syntax than Perl
easier to maintain
does not compete head on with Java
python is a scripting language
whereas Java is a systems language similar to
C++
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Python Modules allow for problem
decomposition
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Alternative import
#!/usr/bin/python
similar to Modula-2
from myfile import title
print title
#!/usr/bin/python
title = "hello world"
above is called myfile.py
#!/usr/bin/python
import myfile
print myfile.title
whereas this file is called hello2.py
when run prints hello world
note that all python modules need to be saved as
name.py
so in our example the module myfile was
saved into a file called myfile.py
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Python builtin types
Builtin objects
python contains many builtin types
use them..
builtin objects make simple programs easy to
understand
lists, dictionaries, exist, don’t reinvent the wheel
numbers
strings
lists
dictionaries
tuples
files
3.14159, 1234
’spam’, "fred’s"
[1, [2, ’three’], 4]
{’food’:’spam’, ’taste’:’yum’}
(1, ’spam’, 4, ’U’)
text=open(’/etc/passwd’, ’r’).read()
built in objects are more efficient than custom data
types
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Expression operators
or, and, not
<, <=, >, >=, ==, <>, !=
x|y
z&y
x << y
x >> y
x[i]
x[i:y]
x.y
x(y)
logical operators (short circuit)
comparison operators
bitwise or
bitsize and
shift left by y bits
shift right by y bits
indexing
slicing
qualifying (imports)
function calls
Strings
concatenation via +
repeated via *
#!/usr/bin/python
print "hi " * 4
yields
hi hi hi hi
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Slicing
Slicing
given a string in Python
first character has index 0
and also -11
last character index is 10 in this example
last character index is also -1
s = "hello world"
you can obtain portion of string via: s[2:5]
yields: llo
negative values start at right and move to the left
strings can be sliced using positive and negative
values
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Statements
assignment, calls, print, if/else/elif, for,
while, break/continue
Statements
class
assert
try, except, raise,
exec
def, return
function definitions and returning values
del
global
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Example 8 times table
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Constructing a simple client and server
the following should be called client.py
#!/usr/bin/python
for n in range(1, 13):
print n, "x 8 =", n*8
#!/usr/bin/python
import sys
from socket import *
serverHost = ’localhost’
serverPort = 2000
# create a TCP socket
s = socket(AF_INET, SOCK_STREAM)
s.connect((serverHost, serverPort))
s.send(’Hello world’)
data = s.recv(1024)
print data
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server.py
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Run the server and client from the
command terminal
#!/usr/bin/python
from socket import *
myHost = ’’
myPort = 2000
# create a TCP socket
s = socket(AF_INET, SOCK_STREAM)
# bind it to the server port number
s.bind((myHost, myPort))
# allow 5 pending connections
s.listen(5)
while True:
# wait for next client to connect
connection, address = s.accept()
while True:
data = connection.recv(1024)
if data:
print "received data", data
connection.send(’echo -> ’ + data)
else:
break
connection.close()
start the server first
$ python server.py
now run the client in a different terminal:
$ python client.py
echo -> Hello world
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Python has many support modules
Python has many support modules
see the global module index in the python online docs
〈http://floppsie.comp.glam.ac.uk/
python/html/index.html〉
read the documentation 〈http://
docs.python.org/library/struct.html〉
and write a small program to print your username in
with 20 characters
take note of the struct module
your program should output
this module provides a list of method which performs
conversions between Python values C objects
01234567890123456789
<fred
>
it can be very usefully employed in constructing
protocols
it uses format strings as compact descriptions of the
layout data
similar to printf and friends in C
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Python has many support modules
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Write a Python program to output your
UID as two bytes in network order
#!/usr/bin/python
#!/usr/bin/python
import struct
import getpass
print " 01234567890123456789"
print "<",
username = getpass.getuser()
print username,
print ">",
# answer
print "<",
print struct.pack("20s", username),
print ">",
the program above pads your username with
character 0’s to make up a 20 character string
$ python teststruct.py
0000000 3020 3231 3433
0000020 3635 3837 0a39
0000040 203c 7266 6465
0000060 0000 0000 0000
0000071
| od
3635
203c
0000
3e20
-x
3837 3039 3231 3433
6167 7569 2073 203e
0000 0000 0000 0000
000a
import struct
import os
print os.getuid()
# answer
print struct.pack("!H", os.getuid())
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TFTP client implementation coursework
support
useful references
rfc1350 〈http://www.faqs.org/rfcs/
rfc1350.html〉
python struct module 〈http://
docs.python.org/library/
struct.html〉
you will need to use the select function in Python
to wait for a tftp packet to discover a timeout
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select
look for the line of code in tftp-skel.py which
matches
r, w, e = select.select([sock], [], [], 5.0)
it follows the behaviour found in select 〈http://
docs.python.org/library/select.html〉
when your code works change this line to
r, w, e = glamnetsim.simselect([sock], [], [], 5.0)
as this function behaves in exactly the same way but
it introduces occasional errors
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Skeleton coursework code
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TFTP client implementation coursework
support
tftp-skel.py 〈tftp-skel.py〉
glamnetsim.py 〈glamnetsim.py〉
useful references
rfc1350 〈http://www.faqs.org/rfcs/
rfc1350.html〉
python struct module 〈http://
docs.python.org/library/
struct.html〉
you will need to use the select function in Python
to wait for a tftp packet to discover a timeout
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select
select
use this function to obtain a packet or wait for a
timeout
when this function returns the values of w and e will
always be [] but the value of r will either be [] or
[sock]
r, w, e = select.select([sock], [], [], 5.0)
it follows the behaviour found in select 〈http://
docs.python.org/library/select.html〉
when your code works change this line to
r, w, e = glamnetsim.simselect([sock], [], [], 5.0)
as this function behaves in exactly the same way but
it introduces the occasional timeout event
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select
to read the packet from the socket you can:
(packet, (address, port)) = sock.recvfrom(512+4)
which reads up to 516 bytes of raw data into packet
and assigns the IP address and port value of the tftp
server
the value [] means a timeout occurred and the
value [sock] means this socket has some data
which is ready to be read